Yesterhaven requires that you own a copy of Dungeon Siege and that your system meets the minimum requirements on the box.
Yesterhaven is an extension to the gameplay and story of The Kingdom of Ehb and the Utraean Peninsula. Yesterhaven is a multiplayer-only short story, which does not require a pre-existing character, nor any previous experience with Dungeon Siege. Yesterhaven can be played in Normal mode as appropriate with new characters, or saved characters from previous Dungeon Siege play.
Once installed, Dungeon Siege: Yesterhaven will be the only map choice in the multiplayer menu, with start locations of Chapter 1, Chapter 2, or Chapter 3. It is strongly recommended that first-time players begin the story at Chapter 1 with a newly created character.